Methods and systems for charging or discharging energy storage systems
A method of allocating power across a microgrid having a plurality of energy storage systems with different power and/or energy capacities, and different states of charge. The method includes allocating a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system.
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The present disclosure relates generally to energy storage systems, and more particularly, to methods and systems for charging and/or discharging energy storage systems.
BACKGROUNDThe present disclosure relates to the control of microgrids that send and receive power. Microgrids may include a plurality of different energy storage systems, such as different battery systems, fuel cells, etc. The different energy storage systems may have different power and energy capacities, and may be at different states of charge. With such varied characteristics between the energy storage systems, charging and discharging the systems in a balanced manner can be difficult. Unbalanced distribution of charge or discharge can affect the health or life of one or more of the energy storage systems, and thus negatively affect the microgrid.
Chinese Patent Application Publication CN110854927A (“the '927 publication”), describes a cooperative control method for an island-type microgrid. The method includes establishing a distributed cooperative communication network to control generators and battery energy storage systems (BESS) of the microgrid to achieve a power balance. The method further includes (1) adopting a consensus algorithm to make the distributed power supply meet the criteria of equal micro-increase rate, that is, the cost of power generation is minimal; (2) using a consistency algorithm, treating the state of charge of each battery energy storage system as a consistency variable, and calculating the average value of the state of charge of the BESSs; and (3) using a p-f droop control method based on the state of charge to adjust the output power of the BESSs so that the state of charge of the BESSs remains balanced. As shown in FIG. 2 of the '927 publication, each BESS first communicates its own state of charge to a neighboring BESS, and each BESS adjusts its state of charge according to its state of charge and the state of charge received from the neighbor. Eventually, the state of charge of each BESS tends to be consistent, corresponding to an average value of the required state of charge. Thus, the method of the '927 publication attempts to balance the state of charge of the BESSs by sharing state of charge information between neighboring BESSs.
The systems and method of the present disclosure may solve one or more of the problems set forth above and/or other problems in the art. The scope of the current disclosure, however, is defined by the attached claims, and not by the ability to solve any specific problem.
SUMMARYThe present disclosure provides for a method of allocating power across a microgrid having a plurality of energy storage systems with different power and/or energy capacities, and different states of charge. The method includes allocating a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system.
In another aspect, the present disclosure provides for a method of allocating power across a microgrid having a plurality of energy storage systems with different power and/or energy capacities, and different states of charge. The method includes allocating a total charge request and/or a total discharge request across the energy storage systems as a function of a current energy, and current state of charge of each energy storage system.
In yet another aspect, the present disclosure provides for a microgrid system including a plurality of energy storage systems having different power and/or energy capacities, and different states of charge; and a microgrid controller communicatively coupled to each energy storage system for sending information to, and receiving information from, each energy storage system. The microgrid controller is configured to allocate a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the features, as claimed. The same reference numbers in different alternatives are used to describe the same components or functions. As used herein, the terms “comprises,” “comprising,” “having,” “including,” or other variations thereof, are intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such a process, method, article, or apparatus. In this disclosure, relative terms, such as, for example, “about,” “substantially,” “generally,” and “approximately” are used to indicate a possible variation of ±10% in a stated value or characteristic.
Any number of energy storage systems 110 could be included in microgrid 100. For example, as shown in
Referring still to
Microgrid controller 140 may include any appropriate hardware, software, firmware, etc. to sense, determine, and/or store various aspect of microgrid 100, and control various aspects of microgrid 100. Microgrid controller 140 may also send information to, and receive information from, each of the plurality of energy storage systems 112, 114, 116, 118 via respective ESS local controllers 122, 124, 126, 128. For example, microgrid controller 140 may receive or determine the need for charging or discharging of power from the microgrid 100, and may be configured to determine and send signals to allocate a total charge request and/or total discharge request across each of the plurality of energy storage systems 110.
As will be discussed in more detail below, when performing the power allocation functions, microgrid controller 140 may allocate a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system 110. The usable energy capacity corresponds to the capacity or amount of energy an energy storage system can receive in response to a total charging request (usable charge energy), or the capacity or amount of energy an energy storage system can discharge in response to a total discharge request (usable discharge energy). The usable charge energy is a function of a maximum state of charge, current state of charge, and current energy of the energy storage system, and the usable discharge energy is a function of a minimum state of charge, current state of charge, and current energy of the energy storage system. Microgrid controller 140 may determine a usable charge/discharge capacity of each ESS 110, a desired charge/discharge of each ESS 110, and a remainder power. While microgrid controller 140 is described as a single controller, it is understood microgrid controller 140 may include multiple controllers such that the functions discussed herein of microgrid controller 140 may be distributed among more than one controller.
Microgrid controller 140 may send and receive information in any appropriate manner, using appropriate hardware, software, firmware, etc. and via wired and/or wireless systems. For example, as noted above, microgrid controller 140 may send and receive information to and from local controllers 120 of each ESS 110 via communication bus 142.
The determination of useable discharge energy for each energy storage system 110 (step 202) may include reading the current energy from each ESS 110, determining the energy content of ESS 110 at an SOC minimum threshold, and determining a usable discharge energy of the ESS 110 as the difference between the current energy and energy at the minimum SOC threshold. As provided in the Equation 1 below, the energy content of ESS 110 at SOC minimum threshold may be determined as an extrapolation from the current SOC of ESS 110 and the SOC minimum threshold. In this equation, and the equations that follow, ESS(i) refers to ESS1 through ESS(n).
Usable Discharge Energy of ESS(i)=Current Energy of ESS(i)−(Current Energy of ESS(i)*(SOC min of ESS(i)/current SOC of ESS(i))) [Equation 1]
In Step 204 of method 200, the determined usable discharge energy of each ESS (112, 114, 116, 118) determined in accordance with Equation 1 is summed to provide a total usable energy of the microgrid 100.
Usable Discharge Energy Total=Sum(Usable Discharge Energy of ESS(i)) [Equation 2]
In Step 206 a desired discharge is determined for each energy storage system 110. This can be determined by multiplying the total desired discharge of the microgrid 100 by the ratio of the Usable Discharge Energy (Equation 1) of an individual energy storage system 110 to the Usable Discharge Energy Total (Equation 2).
Desired Discharge ESS(i)=Total Desired Discharge*(Usable Discharge Energy ESS(i)/Usable Discharge Energy Total) [Equation 3]
When the method 200 moves to step 208—determining a remainder of desired discharge based on discharge limits—the microgrid controller 140 makes sure that the desired discharge of an ESS 110 does not exceed the discharge limit of the ESS 110. If the desired discharge for an ESS 110 would exceed the discharge limit for the ESS 110, microgrid controller establishes a remainder corresponding to the difference between the desired discharge and discharge limit for the ESS 110. See Equation 4 below. As provided in Equation 5 below, the remainders for each of the ESS 110 are then summed to find a reminder total.
If Desired Discharge of ESS(i)>Discharge Limit of ESS(i) then:
Desired Discharge ESS(i)=Discharge Limit of ESS(i) and Remainder of ESS(i)=(Desired Discharge ESS(i)−Discharge Limit(i))
If Desired Discharge of ESS(i)≤Discharge Limit of ESS(i) then:
Desired Discharge ESS(i)=Desired Discharge ESS(i) and
Remainder of ESS(i)=0 [Equation 4]
Total Remainder=Sum(Remainder of ESS(i)) [Equation 5]
Step 210 of
For any ESS(i) where the Desired Discharge<Discharge Limit:
Remaining Desired Discharge=Total Remainder*(Usable Discharge Energy(i)/Usable Discharged Energy Total) [Equation 6]
Desired Discharge(i)=Desired Discharge(i)+Remaining Desired Discharge(i) [Equation 7]
The discharge power allocation method 200 of
While
First iteration desired discharge 410 of ESS4, ESS6, and ESS7 extend the entire height of the bar, indicating that the desired discharge of the first iteration is greater than or equal to the discharge limit 412 of ESS4, ESS6, and ESS7 in accordance with Equation 4 above. Thus, these energy storage systems 110 may provide a remainder and the sum of these remainders corresponds to the total remainder of Equation 6 above. The first iteration desired discharge 410 of ESS1, ESS2, ESS3, and ESS5 is less than their respective discharge limits 412, and thus the total remainder may be allocated in accordance with Equation 6 to these energy storage systems 110. These energy storage systems will have second iteration remaining desired discharges 414, with those of ESS1 and ESS3 meeting or exceeding their respective discharge limits. ESS1 and ESS3 may thus provide another remainder to be allocated in accordance with Equation 6 to ESS2 and ESS5. These energy storage systems will have third iteration remaining desired discharges 416. Since these allocations do not meet the discharge limits for ESS2 or ESS5, then there is no remainder and the third iteration has allocated the total desired discharge. At this point, the discharge request sent from microgrid controller 140 to the local controllers 120 of each ESS 110 can be determined in accordance with Equation 7.
The bar chart of
The previous discussion of
The determination of useable charge energy for each energy storage system 110 (step 602) during charge power allocation may include reading the current energy from each ESS 110, determining the energy content of ESS 110 at an SOC maximum threshold, and determining a usable energy of the ESS 110 as the difference between the current energy and energy at the maximum threshold. As provide in the equation below, the energy content of ESS 110 at SOC maximum threshold may be determined as an extrapolation from the current SOC of ESS 110 and the SOC maximum threshold. In this equation, and the equations that follow, ESS(i) refers to ESS1 through ESS(n).
Usable Charge Energy of ESS(i)=Current Energy of ESS(i)−(Current Energy of ESS(i)*(SOC max of ESS(i)/SOC of ESS(i))) [Equation 8]
In Step 604 of method 600, the usable charge energy of each ESS 112, 114, 116, 118 is summed to provide a total usable charge energy of the microgrid 100.
Usable Charge Energy Total=Sum(Usable Charge Energy of ESS(i)) [Equation 9]
In Step 606 a desired charge is determined for each energy storage system 110. This can include multiplying the total desired charge of the microgrid 100 by the ratio of the usable charge of an individual energy storage system 110 to the total usable charge energy.
Desired Charge ESS(i)=Total Desired Charge*(Usable Charge Energy ESS(i)/Usable Charge Energy Total) [Equation 10]
When the method 600 moves to step 608—determining a remainder of desired charge based on charge limits—the microgrid controller 140 makes sure that the desired charge of an ESS 110 does not exceed the charge limit of the ESS 110. If the desired charge for an ESS 110 exceeds the charge limit for the ESS 110, microgrid controller establishes a remainder corresponding to the difference between the desired charge and charge limit for the ESS 110. The remainders for each of the ESS 110 are then summed to find a reminder total.
If Desired Charge of ESS(i)>Charge Limit of ESS(i) then:
Desired Charge ESS(i)=Charge Limit of ESS(i) and
Remainder of ESS(i)=(Desired Charge ESS(i)−Charge Limit(i))
If Desired Charge of ESS(i)≤Charge Limit of ESS(i) then:
Desired Charge ESS(i)=Desired Charge ESS(i) and
Remainder of ESS(i)=0 [Equation 11]
Total Remainder=Sum(ESS(i)) [Equation 12]
Step 610 of
For any ESS(i) where the Desired Charge<Charge Limit:
Remaining Desired Charge=Total Remainder*(Usable Charge Energy(i)/Usable Charged Energy Total) [Equation 13]
Desired Charge(i)=Desired Charge(i)+Remaining Desired Charge(i) [Equation 14]
The disclosed methods and systems for charging and/or discharging energy storage systems may be used in any microgrid-type system that sends and receives power.
The methods and systems disclosed herein may provide a balancing of state of charge (SOC) and power distribution across energy storage systems 110 of a microgrid 100 when the energy storage systems 110 of the microgrid 100 have different power and/or energy capacities and different states of charge. The method and systems allow for microgrid controller 140 to provide asymmetric charge or discharge requests as a function of usable capacity of each energy storage system 110. With this, the systems and methods disclosed herein may allow the SOC of each energy storage system 110 to eventually converge when the rated capacities of energy storage systems are different. Providing such a balancing of the SOC of the energy storage systems 110 may extend the health and life of the systems, and thus may extend the health and life of the microgrid 100.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed machine without departing from the scope of the disclosure. Other embodiments of the machine will be apparent to those skilled in the art from consideration of the specification and practice of the system and methods described herein. For example, the above described process steps need not be performed in the order described, but rather certain steps can be performed in a different order and/or can be performed simultaneously with other steps. Further, it is understood that one or more of the energy storage systems 110 of microgrid 100 may be excluded from the charge or discharge allocations provided above. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.
Claims
1. A method of allocating power across a microgrid having a plurality of energy storage systems with different power and/or energy capacities, and different states of charge, the method comprising:
- determining a usable charge energy for each energy storage system, wherein the usable charge energy is determined based on a difference between current energy and energy content at a state of charge maximum threshold of each energy storage system;
- determining a usable discharge energy for each energy storage system, wherein the usable discharge energy is determined based on a difference between the current energy and the energy content at a state of charge minimum threshold of each energy storage system;
- determining a total usable charge energy and/or a total usable discharge energy of the microgrid based on the usable charge energy and the usable discharge energy of the energy storage systems; and
- allocating a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system.
2. The method of allocating power of claim 1, wherein the usable energy capacity is a capacity of the energy storage system to receive energy in response to the total charge request or a capacity of the energy storage system to discharge energy in response to the total discharge request.
3. The method of allocating power of claim 1, wherein the allocation is based on a ratio of the usable energy capacity of each energy storage system to a total usable energy capacity of all of the energy storage systems combined.
4. The method of allocating power of claim 1, wherein the allocation is also a function of a charge limit when allocating the total charge request, and the allocation is also a function of a discharge limit when allocating the total discharge request.
5. The method of allocating power of claim 1, wherein the energy storage systems include at least one of a battery system or a fuel cell system.
6. The method of allocating power of claim 1, further including:
- determining the usable energy capacity of each energy storage system based on a current energy and current state of charge of the energy storage system.
7. The method of allocating power of claim 6, further including:
- determining a desired charge for each energy storage system when allocating the total charge request; and
- determining a desired discharge for each energy storage system when allocating the total discharge request.
8. The method of allocating power of claim 7, further including:
- determining a remainder of the desired charge for each energy storage system when allocating the total charge request; and
- determining a remainder of the desired discharge for each energy storage system when allocating the total discharge request; and
- the remainder of the desired charge is a function of a charge limit of the energy storage system, and the remainder of the desired discharge is a function of a discharge limit of the energy storage system.
9. The method of allocating power of claim 8, further
- allocating the remainder of the desired charge across unsaturated energy storage systems until the total charge request is met; and
- allocating the remainder of the desired discharge across the unsaturated energy storage systems until the total discharge request is met.
10. The method of allocating power of claim 9, further including:
- sending a desired charge request to each energy storage system as a sum of the desired charge and any remainder of the desired charge; and
- sending a desired discharge request to each energy storage system as a sum of the desired discharge charge and any remainder of the desired discharge.
11. The method of allocating power of claim 10, wherein the microgrid allocates both the total charge request and the total discharge request.
12. The method of allocating power of claim 1, wherein distributing power from the energy storage system is asymmetric across the energy storage systems to balance the state of charge and power distribution between the energy storage systems.
13. A method of allocating power across a microgrid having a plurality of energy storage systems with different power and/or energy capacities, and different states of charge; the method comprising:
- determining a usable charge energy for each energy storage system, wherein the usable charge energy is determined based on a difference between current energy and energy content at a state of charge maximum threshold of each energy storage system;
- determining a usable discharge energy for each energy storage system, wherein the usable discharge energy is determined based on a difference between the current energy and the energy content at a state of charge minimum threshold of each energy storage system;
- determining a total usable charge energy and/or a total usable discharge energy of the microgrid based on the usable charge energy and the usable discharge energy of the energy storage systems; and
- allocating a total charge request and/or a total discharge request across the energy storage systems as a function of a current energy, and current state of charge of each energy storage system.
14. The method of allocating power of claim 13, wherein the allocation of the total charge request is also a function of a maximum state of charge of the energy storage system; and the allocation of the total discharge request is also a function of a minimum state of charge of the energy storage system.
15. The method of allocating power of claim 14, wherein the allocation is based on a ratio of a usable energy capacity of each energy storage system to a total usable energy capacity of all of the energy storage systems combined, and wherein the allocation is asymmetric across the energy storage systems.
16. The method of allocating power of claim 15, wherein the allocation is also a function of a charge limit when allocating the total charge request, and the allocation is also a function of a discharge limit when allocating the total discharge request.
17. The method of allocating power of claim 16, wherein the energy storage systems include at least one of a battery system or a fuel cell system.
18. The method of allocating power of claim 15, wherein distributing power from the energy storage system is asymmetric across the energy storage systems to balance the state of charge and power distribution between the energy storage systems.
19. A microgrid system, comprising:
- a plurality of energy storage systems having different power and/or energy capacities, and different states of charge;
- each of the plurality of energy storage systems includes a local controller that is communicatively coupled to a power bus for receiving and discharging a charge; and
- a microgrid controller communicatively coupled to the local controller of each of the plurality of energy storage system across a communication bus for sending information to, and receiving information from, each of the plurality of energy storage systems, wherein each of the plurality of energy storage systems includes power electronics to provide an interface between the energy storage system and the power bus and the energy storage system and the communication bus, the microgrid controller configured to: allocate a total charge request and/or a total discharge request across the energy storage systems as a function of a usable energy capacity of each energy storage system.
20. The microgrid system of claim 19, wherein the microgrid controller is further configured to allocate both the total charge request and the total discharge request, and:
- determine the usable energy capacity of each energy storage system based on a current energy and current state of charge of the energy storage system;
- determine a desired charge for each energy storage system when allocating the total charge request, and determine a desired discharge for each energy storage system when allocating the total discharge request;
- determine a remainder of the desired charge for each energy storage system when allocating the total charge request, and determine a remainder of the desired discharge for each energy storage system when allocating the total discharge request; wherein the remainder of the desired charge is a function of a charge limit of the energy storage system, and the remainder of the desired discharge is a function of a discharge limit of the energy storage system;
- allocate the remainder of the desired charge across unsaturated energy storage systems until the total charge request is met, and allocate the remainder of the desired discharge across the unsaturated energy storage systems until the total discharge request is met; and
- send a desired charge request to each energy storage system as a sum of the desired charge and any remainder of the desired charge, and send a desired discharge request to each energy storage system as a sum of the desired discharge charge and any remainder of the desired discharge.
21. The method of allocating the power of claim 7, further comprising:
- determining the desired discharge by multiplying total desired discharge of the microgrid by a ratio of the usable discharge energy to the total usable discharge energy.
22. The method of allocating the power of claim 21, further comprising:
- determining the desired discharge exceed a discharge limit of at least one energy storage system; and
- determining a remainder corresponding to a difference between the desired discharge and the discharge limit for the energy storage systems, wherein the remainder is allocated to at least one other energy storage system that has not reached its discharge limit.
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Type: Grant
Filed: Oct 27, 2020
Date of Patent: Aug 29, 2023
Patent Publication Number: 20220131381
Assignee: Caterpillar Inc. (Peoria, IL)
Inventors: Kevin J. Sergott (Dunlap, IL), Ronald C. Gayles (Peoria, IL)
Primary Examiner: Brian Ngo
Application Number: 17/080,962
International Classification: H02J 7/00 (20060101); H02J 3/32 (20060101);